Currently, clinical diagnosis of skin disease is generally accomplished by visual inspection under white light illumination. In this process, the reflectance light of a skin lesion is examined. Visual diagnosis alone may not be particularly accurate for early detection of skin cancer since many skin conditions have a similar appearance under white light. Therefore, when a suspect lesion is identified by visual examination, a biopsy is often performed for a definitive diagnosis. Not only is it crucial to diagnose skin pre-cancer or skin cancer at an early stage when it is curable, but it is also important to improve the clinical diagnosis of suspected skin lesions so as to avoid unnecessary skin biopsies.
Several approaches have been tried to improve dermatologic diagnosis. Digital processing of reflectance images has been extensively investigated recently. Although reflectance imaging has led to improvements in the registration, recording, and documentation of skin lesions, there has been little improvement in the diagnostic accuracy. The foregoing approach does not provide any additional data to the physician making the visual assessment because it is still based on the reflectance pattern of a lesion under white light illumination which is essentially the same pattern a human observer sees.
An alternative approach is ultraviolet (UV) or infrared (IR) photography which does extend visual perception of a physician to the UV or IR reflectance patterns. However, the inconvenience due to delays in processing of film images renders this technique impractical for everyday use.
A further alternative approach that is already in widespread medical use involves a "Wood's lamp" which consists of a mercury discharge lamp associated with a filter that transmits UVA light with a 365 nanometer peak while absorbing visible light. When this device is used to assist in skin diagnosis, the eye serves as both the detector and the long pass filter. The eye is not sensitive to UV light, but is sensitive to visible fluorescence light when the "Wood's lamp" is used in a darkened room, where the physician sees an image of a fluorescing disease site. The "Wood's lamp" is useful for the diagnosis of some skin conditions such as tinea capitis, tinea versicolor, erythrasma, and some pseudomonas infections, as well as aiding in the detection and diagnosis of hypopigmented skin. It is of no value in conditions where the fluorescence is not in the visible spectrum.